Zoroastrianism has often been portrayed as a religion that has embodied an absolute form of dualism, a doctrine that recognizes existence to be under the domain of two antithetical principles. While a form of dualism between righteousness and falsehood is echoed throughout the Avesta, the Gathas of Zarathushtra present an ethical rather than a cosmic dualism. In the latter, forces designated as good and evil are said to operate on both spiritual and corporeal levels, but in Zarathushtra's view the world is definitely not bifurcated into all things good and evil.

Is there a definite norm against which good or evil can be characterized? Unfortunately, defining good and evil is not simple because getting it wrong means either punishing the good or rewarding evil or furthering it. A quick trip to the dictionary yields the following basic terms:
Good - Being virtuous, having desirable qualities, behavior that is altruistic, intentionally beneficial to others more than to oneself, respect and concern for the dignity of sentient beings;
Evil - Morally bad or wrong, causing ruin, injury or pain, selfishness and lack of concern for others, debase or destroy innocent life for fun or personal gain.

In his Gathas, Zarathushtra has attributed the origin of evil to the human mind.

Zarathushtra is believed to have lived sometime between 1500 and 1200 BCE when belief in many Gods (Sanskrit: Asuras, Avestan: Ahuras) prevailed. Also, the concept of right and wrong, namely, Asha, (Sanskrit: Rta) symbolizing truth and righteousness, and Drug, representing falsehood, were already present at that time. Zarathushtra brought forth a profound vision, a radically new idea rooted in wisdom and conscience, constituting a novel view of the world and a unique way of life. He elevated Ahura Mazda, the Lord of Wisdom, to the position of the only God, and associated him with Asha, which has many connotations such as goodness, truth and righteousness.

Physicists Stephen Hawking and Roger Penrose have proved that on the cosmic scale, time cannot extend back indefinitely for as we go backward in time, the galaxies all come together to a single infinitesimal point, known as singularity. The singularity is the ultimate cataclysm, beyond which our cosmic ancestry cannot extend. However, the recently developed String Theory suggests that the Big Bang was not the origin of the universe but simply the outcome of a preexisting state. On the other hand, John Polkinghorne, a physicist and the winner of the $1-million Templeton prize, the equivalent of the Noble Prize in religion, mentioned during his award ceremony: “we live in a world that is rationally beautiful and does indeed look as though it is shot through with signs of mind. So it's quiet natural to ask whether this might not be due to the fact that there is a capital M mind behind it all. Creation is not just some performance of a fixed score that God wrote in eternity. It is a sort of an unfolding process. Theologically, you understand an evolving world as a Creation that is endowed by its creator with fruitful potential but is then allowed to make itself."

According to Zarathushtra, Ahura Mazda is the creator and promoter of the universe. Ahura Mazda conceived the universe, manifested it through his creativity (Spenta Mainyo), and set it in motion in accordance with Asha to promote the advancement of his creations towards a better existence. Everything that conforms to Asha is good and the law of Asha regulates the changes in the universe, maintains world dynamism and is manifested in the laws of nature with its cycle of birth, growth, decay and rejuvenation. A universe where death, decay and natural recycling do not occur would be a very static universe without much possibility for diversity, change or growth. Dastur Dhalla [1], eloquently states: “Life of all joy and happiness, with never a shadow of sorrow and misery, would be monotonous and would weary us.” Asha represents absolute values and provides the standards that apply to all people at all times. Ahura Mazda, the fashioner of the Universe is all good and he can only do good things and the opposite of goodness cannot be attributed to Him. Then, how could a benevolent and all wise creator permit the existence of evil?

To resolve this let us first look at Ahunavaiti Gatha’s Yasna 30.2 where Zarathushtra states:

"Listen with your ears to the noblest truths;
ponder (over) with your illumined Intelligence the tenets of your (own) choice,
every man for his own self.
Indeed awaken each one to illume himself, before the mighty Consummation

English translation by Rustomjee [2]

Zarathushtra tells us that Ahura Mazda created human beings with a free choice to make moral judgments and act on them. However, this does not imply that a person is justified in doing whatever he/she pleases. It only means that every person has to make the choice and bear the responsibility, which one cannot avoid under cover of higher command, divine injunction or obligatory submission to some power. Everyone is born with a Manah (mind) which can be used in either a good or bad way or a combination of both. Of course, certain physical factors beyond the control of some individuals, such as genetic weaknesses, can have a significant effect on one’s mental development. Mentally challenged persons along with animals and non-sentient creatures are neutral because they lack the capacity to distinguish between morally right and wrong behavior. With the exception of human beings, most creatures live by the Divine instincts inherent in them and they survive or perish by following the laws of nature. These creatures do not have the gift to reason, rationalize or articulate. Zoroastrianism is the religion of personal individual choices par excellence and that is what makes it unique. Ahura Mazda will not force a human being to make the right choice or interfere, by coercion and bribes, with the person’s free will to choose. The ability to decide what is good and what is evil, is the function of understanding. The better the understanding, the greater is the person’s ability to recognize right from wrong; and wisdom can be defined as the ability to distinguish between good and evil. Every person must use reason, intuition and logic so faith is not totally blind.

Thus in the beginning, the two mental aspects, which are twins
Mutually disclosed themselves in their thoughts, words and deeds,
The one as the better (of the two) and the other (as) the evil
The wise and the intelligent did choose correctly but the ignorant and unwise did not.

English translation by Rustomjee [2]

In Yasna 30.4, Zarathushtra reveals that the two mental aspects were not separate and distinct but the product of the one and the same mind. When they came together they brought about Gaya, life or illumination of the mind and Ajyaitim absence of life or confusion of the mind. Thus, Gaya is the light and life of the mind but has often been erroneously interpreted as life, as in creation. The Avestan word for life that is created is Anghahu [2].

Zarathushtra refers to the two mental aspects as ýęmâ (twins). It implies that it is impossible to conceive of one without the other and at the moment we become aware of the good, that very moment we become aware of the other. Evil remains a possibility until some action makes it real and when free-willing individuals, through their own rational choice, do not choose evil, then evil losses its influence, it is destroyed. Zoroastrianism is a religion which promotes a modern, intelligent and sane way of thinking and behavior. Good thoughts, good words and good deeds feed righteousness. According to Dastur Dhalla [1]: “He lives his life best upon earth, who lives in righteousness and for righteousness.” Wrongful choices diminish Asha in our world and retard the purpose of life which is to further the cause of the physical world and make it a better place for this generation and for generations to come.

In his Gathas, Zarathushtra refers several times to Ahura Mazda and his creative mentality Spenta Mainyo but never to their antithesis; and only once the term akascâ mainyush, meaning wicked mentality, appears in Yasna 32.5. However, the texts of the Younger Avesta explain the Twins as Ahura Mazda and Anghra Mainyu (devil) emanating from a supreme deity, Zurvan, the deity of time. Over the centuries, the reverence to Zurvan receded but the coequal nature of the two entities has remained and given rise to the cosmic duality. Many Zoroastrians still believe that there is a constant struggle in the universe, a cosmic battle between the two primal principles, Ahura Mazda representing light and Anghra Mainyu representing darkness. These two entities are supposed to be in battle with one another not only on our earthly plane but also in a cosmic sense. This is why the Greek philosophers, Plato and Aristotle, refereed to the Zoroastrianism as the religion of two Gods, Oromazdes and Areimanios [3]. The Gathas of Zarathushtra do not propose a total partition of the cosmos into two groups of good and evil; but employ ethical dualism to explain the situation of the human beings in this world. In Yasna 30.5 Zarathushtra clearly attributes the origin of evil to the evil minded (dregvĺ) i.e. the people who selected (varatâ) evil over good.

Zarathushtra teaches his disciples to be active and constructive because monasticism, celibacy, asceticism and self-mortification have no place in Zoroastrianism. We should not denigrate the material world but act wisely and conserve to preserve it. With acquired knowledge and advancements in health sciences and technology we can take better care of our health, stay active and fight injustice and evil whenever we have the chance. We should eradicate falsehood for the advancement of the world and the progress of human beings towards betterment. The triumph of good over evil is gradual and not abrupt. One cannot just instantaneously rid the world of all its miseries but each one of us can play a role in destroying evil. Until recently, there was a general consensus among scientists that our genetic makeup was hard-wired into our DNA. The new field of epigenetics suggests that certain chemicals attach themselves to our DNA to form a second code of programming on top of our DNA and change the way our genes function. Unlike DNA, this second code can change during our lifetimes and some of these changes can be passed on to our children and grandchildren. The field of epigenetics adds a whole new moral dimension to our own behavior and suggests that our behavior can change the way the genes of our prodigy can function. By choosing not be violent ourselves, by choosing to help people when we can, by choosing to make beautiful things which bring joy to this world, we can make a difference. By implementing Asha we can eliminate evil and make ourselves and our world move towards Haurvatat (completeness). Human beings are co-workers of Ahura Mazda, but not his slaves. The beauty of the Zoroastrianism is that human beings, by making the right choices with their own free will, can become like Ahura Mazda and in the process get rid of evil from this world.